Everyone needs vitamin D, which is essential for healthy bones and teeth, but it's also known to have a protective role against a wide range of degenerative diseases.

Known as the 'sunshine vitamin', 90 per cent of your body's supply comes from sunlight. We ask experts, how much sunshine is enough?

Why is it beneficial?

Of all the nutrients we need: vitamin D has been subject to most debate recently, with reports of increasing incidents of vitamin D deficiency and cases of the childhood illness rickets – a disease many thought was confined to the pages of history.

Just as deficiency appears to be on the rise, there's also growing awareness of how important it is for a whole host of bodily functions.

'Over the last few years scientists have begun to realise how important vitamin D is, not just for bone and tooth development but for a large number of other bodily activities too,' says Dr Susan Lanham, head of the nutritional sciences division at Surrey University.

'We have 200 genes in our bodies that require vitamin D to function.

'A prime example is that the cells in our pancreas, which are involved in the production of insulin, need vitamin D. We're now discovering a strong link between vitamin D deficiency and the risk of developing diabetes,' she says.

Sunlight

Strictly speaking vitamin D is not actually a vitamin – it's a prohormone, an inactive precursor of a hormone.

It's made in the skin through absorption of sunlight. It's the only nutrient we need that we don't get mostly from diet.

Much recent debate about vitamin D has centred on whether sun avoidance messages have contributed to growing levels of vitamin D deficiency.

'Whether levels of vitamin D deficiencies are growing, or whether it's simply because we are studying it more, we don't really know,' says Dr Lanham.

Typically immigrant populations are thought to be at risk of vitamin D deficiency because dark skin is less able to produce vitamin D and because in some cultures women cover up.

However a study led by Dr Veronique Bataille, a consultant dermatologist and skin cancer specialist at Hemel Hempstead Hospital, Spire Hospital Harpenden and the Princess Grace Hospital, also found that those with fair skin and red or blonde hair, had surprisingly low levels of vitamin D.

'The difficulty with the sun avoidance message is that it's a blanket message that equates sun exposure with melanoma and skin cancer,' says Dr Bataille.

'We don't want people to swing the other way and say what we have learnt about sun awareness in the last 15 years isn't true. But if you have olive skin and tan well, you shouldn't be paranoid.

'However, if you're fair skinned or concerned about skin ageing, you might want to think about taking a supplement,' she says.

How much sun?

There's currently no daily recommended intake of vitamin D in the UK.

But around 20 micrograms would provide a sufficient amount. This equates to around 15 to 20 minutes of sunlight a day, without sunscreen and with 10 per cent of your body surface uncovered – roughly that's equal to your face, neck, hands and lower arms.

As a rule of thumb, your body will only make vitamin D when your shadow is shorter than your height.

'Sunscreen will inhibit about 95 per cent of vitamin D production. Glass windows – in cars, homes and offices – will also block the sun's rays. But you only need a small amount,' says Dr Lanham.

'Your body is smart and will stop making vitamin D after about 30 minutes, which is before vitamin D toxicity,' she says.

However, between October and March there's not enough sunlight in the UK to make vitamin D.

'It used to be thought that you made enough vitamin D in the summer to last you through the winter. But according to a large study (as yet unpublished) we're all deficient in vitamin D during the winter,' says Dr Lanham.

Spotting deficiency

People with low levels of vitamin D may experience extreme tiredness, muscle pain and weakness and bone aches.

In severe cases it can cause osteomalacia, where the bones become weakened and soft.

In children vitamin D is vital for both tooth and bone development, and deficiency can lead to soft skull and leg bones, poor growth and tooth delay, proneness to infection, respiratory problems and in severe cases rickets – where sufferers typically develop a curved, bow-legged look to their legs.

Food sources

Vitamin D is not found in many foods. But if you aren't getting enough sun, try to include foods that include vitamin D in your diet.

These include:

oily fish – salmon, tuna, mackerel, anchovies

shellfish

eggs

liver

fortified cereals

margarines.

However, remember the amount of vitamin D that can be obtained from food is low, around 2 to 3 micrograms a day.

Supplements

Vitamin D supplements can be useful, especially in winter when a large number of people are typically low on the sunshine vitamin.

Pregnant mothers or those trying to conceive should particularly consider taking a supplement during the winter.

Sun care

Advice about taking care in the sun remains, and you should avoid burning.'

Excessive sun exposure is the main cause of skin cancer, including malignant melanoma, the fastest rising type of cancer in the UK,' says Sara Hiom, Cancer Research UK's director of health information.

'Between 11 and 3pm the sun's rays are strongest and most damaging. Our advice therefore remains to spend some of that time in the shade or protected from the sun.

'How long a person can safely stay in the sun will depend on their skin type – the fairer the skin the shorter the time they should spend in the sun,' she says.

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